1. Field of the Invention
This invention relates to a transformer for transferring electrical power into a machine member that is rotating at high speed.
2. Description of Related Art
In rotating mechanical systems there is often a need to transfer electrical power into a load mounted on a rotor operating at high rotational speeds.
A common method of power transfer is magnetic induction. A magnetic field is created with a stator. Movement of a coil mounted on the rotor through the magnetic field created by the stator generates an induced current in the coil according to Lenz's Law. The amount of power transferred is relative to the intensity of the magnetic field and the speed of rotor with respect to the stator. For low power transfer the magnetic field may be created by permanent magnets in the stator. For higher power transfer electromagnets are typically used to create the magnetic field.
The rotor speed determines the amount of power transferred into the rotor. As the electrical load on the rotor changes so does the torque load. This may cause the rotational speed of the rotor to change. Independently controlling the power delivered to the load in the rotor without affecting the rotational speed of the rotor thus presents practical problems.
One method of controlling the power delivery to an electrical load in a rotor independent of rotor speed is by the use of slip rings. Slip rings are typically mounted in an electrically insulated manner on the rotor and employ brushes mounted in an electrically insulated manner on the fixed stator element of the machine. The brushes are typically made of a relatively soft electrically conductive material, such as graphite, and are spring-loaded to maintain adequate contact with the slip rings. The brushes have limited service life due to wear while in use and must be periodically replaced. The rate of wear is a function of both the surface speed of the slip ring against the brush and the current being transferred. The surface speed is a function of the rotational speed and the slip ring diameter. An increase in surface speed accelerates the rate of brush wear. At high current levels and high surface speeds the slip rings tend to erode and must be periodically refurbished.
Due to the limited power transfer achievable, reliability, and environmental issues, slip ring/brush components have been eliminated from many applications.
At low alternating current frequencies, such as fifty (50) or sixty (60) Hertz line frequencies, ferrite or other high coercivity materials may be efficiently used within both the stator and the rotor to couple power into a rotating load. However, to transfer significant amounts of power such high coercivity materials must be physically large and heavy. Large heavy masses of ferrite in the rotor can cause significant dynamic mechanical problems when the rotor is rotated at high speed.
Accordingly, in view of the foregoing, it is believed advantageous to provide a power transfer arrangement for transferring electrical power into a high speed, continuously operating, rotating member in an efficient and maintenance-free manner without introducing undesirable dynamic balance problems.